In many arid regions like Saudi Arabia, the near surface velocity of seismic waves in the shallowest few hundred meters varies significantly due to variations in the rocks encountered in these shallow geological layers. Such variations include, for example, sand dunes, buried caverns filled with sand and the sporadic occurrence of high velocity anhydrite lenses. Because wells drilled through these shallow layers are not generally logged, these heterogeneities are not sufficiently sampled and their effects are unaccounted for in correcting 3D surface seismic data. This often results in a distortion of the image of the objective horizons at the reservoir zones obtained from 3D surface seismic recording. Due to such distortions or false structures, many wells for field development have been drilled at poor locations caused by local near-surface velocity anomalies.
Compensating for these anomalies is an extremely challenging problem. In the past, when only 2D seismic data were acquired, shallow “up-holes” were drilled along the 2D lines. Explosives detonated on the surface or, alternatively, a mechanical source like a weight drop provided the seismic source that was detected in geophones or seismic sensors lowered to various measured depths in the drilled up holes. This experiment provided a time delay for the seismic source to be detected at the receivers. These time-depth measurements sampled the near surface velocity variations at drilled up-hole locations. From these measurements, near surface velocity models were constructed along the 2D seismic line profiles where several upholes were drilled at regular distances from each other.
This is an expensive, albeit effective, tool for measuring the velocity in the shallow layers. It is the extensive areal coverage of modern 3D surveys that makes adequate uphole control that is necessary for shallow velocity modeling very expensive. Accordingly, there is a need for a more economical technique to sample the velocities.
There are known techniques called “seismic while drilling” (SWD) technology. These techniques and their corresponding equipment have been developed to record seismic data produced by the drill bit as it penetrates the rocks. The use of seismic waves generated by a drill bit while drilling a borehole has been disclosed in various U.S. patents, e.g. U.S. Pat. Nos. 2,062,151; 4,954,998; 4,964,087; 4,965,774; 5,248,857; and 5,511,038.
In the previously described seismic source-receiver systems, the time delay is recorded measured from the initiation of the source. However, in SWD the seismic signal produced by the drill bit in the borehole is a continuous signal that propagates through the earth formations and is detected by an array of sensors on the ground surface. The continuous signal has no reference zero time break.
However, the continuous drill bit signal also propagates along the steel drill string assembly to the surface and this can be detected using accelerometers mounted on a swivel joint above the drill pipe. This has been discussed in U.S. Pat. No. 4,718,048 and European Patent 0273 722. Also, U.S. Pat. Nos. 4,365,322 and 5,050,130 describe the use of these continuous seismic signals from the drill bit in data processing.
The zero time or the instant of emission of seismic elastic waves from the drill bit is obtained by comparing the signals recorded on the drill-string with those recorded on the surface sensors. These two signals propagate along two distinct paths. Processing consists of cross correlating the signal propagating through the earth layers with the signal propagating along the drill string.
Under any of the prior art discussed above, the seismic signals emanated from a working drill bit have not been applied for providing weathering, sub weathering or the near surface layering model or in deriving a tomographic velocity model for the correction of surface 3D seismic data. The joint inversion of different elastic wave types from SWD data for near-surface characterization is hitherto unknown.